1. Field of the Invention
The present invention relates to a shrink fitting method, and more particularly, to a new shrink fitting method including deformation, in which a high bonding force can be obtained without accurate mechanical process such as forming an insertion body having an outer diameter greater than an inner diameter of an object for receiving the insertion body.
2. Description of the Background Art
Shrink fitting is metal bonding technique using expansion and shrinkage of metal due to heat, and is widely used for mechanical parts owing to advantages of mechanical bonding with no welding defect and simple process.
The principle of shrink fitting will be described as follows. For example, supposing that a metallic rod material is bonded into a pipe, the pipe is prepared to have an inner diameter smaller than a diameter of the metallic rod material. If the pipe is heated, its inner diameter increases due to thermal expansion, so that the rod material can be inserted into the pipe. Then, if the pipe is cooled at a normal temperature, the inner diameter of the pipe should be smaller than the diameter of the rod material. In this case, a strong bonding force occurs between the pipe and the rod material. Thus, the rod material is strongly fitted into the pipe even though the inner diameter of the pipe is smaller than the diameter of the rod material.
The principle of shrink fitting is very simple but has difficulty in practical use due to accuracy. How the inner diameter of the pipe should be smaller than the rod material and its relevant problem will be described with reference to FIG. 1. FIG. 1 illustrates a bonding force based on a difference between the inner diameter (ID) of the heated pipe and the diameter (OD) of the rod material. Three fields can be formed by the difference. If the difference is greater than an increment of the inner diameter due to thermal expansion, there is no bonding force. This is because that the pipe is just shrunk (field I). In this case, the pipe is prepared to have an inner diameter greater than the diameter of the rod material at a normal temperature.
In the second field, the bonding force increases if the diameter difference increases. This is because that restricted thermal expansion (i.e., not returned to original state even in case of cooling) increases (field II). If there is no diameter difference, the bonding force reaches a maximum value. In this case, the rod material is tightly fitted into the heated pipe.
If the OD is greater than the ID as shown in the field 111, there is no bonding as the rod material is not inserted into the pipe.
Generally, it is known that an industrial pipe or rod material has a diameter tolerance in the range of 2% because it is not a genuine circle. Since the change of the diameter is in the range of 0.5% (0.45% if heated at 300° C. because thermal expansion coefficient of metal is generally 15×10−6/° C.) due to thermal expansion, shrink fitting is not suitable for the industrial pipe or rod material.
Thus, shrink fitting cannot be widely used for general industrial parts because it is available only when accurate mechanical process follows.